With the recent industrial trend toward labor saving or toward miniaturization, weight reduction, and speed increase in machinery, there is a growing desire for a high-grade lubricating grease having a combination of good performance characteristics, such as heat resistance, load carrying capacity, anti-wear property, and a long life, for use in application to the bearings and gears of such industrial machinery. For example, the lubrication of CVJs, which are frequently employed in front-drive vehicles, necessitate a lubricating grease excellent in durability, anti-wear property, and heat resistance, because of the trend toward power and speed increase in motor vehicles and weight reduction in CVJ's themselves.
In ironworks, on the other hand, rolling mills have gradually shifted to higher-speed mills or mills having sealed bearings, as a result of the simplification and modernization of equipment. Because of this, lubricating greases for this use are not only strongly required to contribute to energy saving, but also required to have a combination of good performance characteristics including load carrying capacity, heat resistance, and a long life. Thus, the performance characteristics required of lubricating greases for use in various industrial fields including the automobile and the steel industries are becoming more severe with the progress of machines. The main demand of greases for machinery has been shifting from lithium soap greases called "universal greases" or "multipurpose greases" to urea greases, which have excellent heat resistance and are capable of producing a synergistic effect with an additive to attain a reduction of friction.
Under these circumstances, representative techniques are described in, e.g., U.S. Pat. Nos. 4,840,740, 4,514,312, and 4,787,992. U.S. Pat. No. 4,840,740 discloses a grease composition comprising a urea grease containing a combination of an organomolybdenum compound and zinc dithiophosphate. U.S. Pat. No. 4,514,312 discloses a grease composition comprising a urea grease containing an aromatic amine thiophosphate. U.S. Pat. No. 4,787,992 discloses a grease composition comprising a grease thickened with a Ca soap, a Ca-complex soap, or a mixture of any of these with a urea compound and containing a combination of calcium carbonate and tricalcium phosphate as additives. JP-B-4-34590 (the term "JP-B" as used herein means an "examined Japanese patent publication") discloses a composition comprising a urea grease containing, as an essential component, a sulfur-phosphorus extreme-pressure additive comprising a combination of (A) a sulfurized molybdenum dialkyldithiocarbamate and (B) at least one member selected from the group consisting of sulfurized fats and oils, sulfurized olefins, tricresyl phosphate, trialkyl thiophosphates, and zinc dialkyldithiophosphates. Further, JP-B-4-65119 discloses a lubricating grease composition obtained by kneading a mixture of a base oil and tricalcium phosphate, which is represented by the formula Ca.sub.3 (PO.sub.4).sub.2.
However, these patented compositions have drawbacks as follows. The compositions based on a urea grease each is still insufficient in load carrying capacity and heat resistance, although it is expected that the grease and the additive(s) produce a synergistic effect to reduce friction. On the other hand, the tricalcium phosphate grease has a drawback that it is slightly inferior in anti-wear property to the urea greases although superior in load carrying capacity and heat resistance.